Gas turbine engines produce power by extracting energy from a flow of hot gas produced by combustion of a fuel-air mixture. In general, gas turbine engines have an upstream air compressor coupled to a downstream turbine with a combustion chamber (“combustor”) in between. Energy is released when a mixture of compressed air and fuel is ignited in the combustor. The resulting hot gases are used to spin blades of the turbine, and produce mechanical power. In typical turbine engines, one or more fuel injectors direct some type of fuel (liquid or gaseous fuel) into the combustor for combustion. The fuel is mixed with compressed air in the fuel injector, and delivered to the combustor for combustion. Combustion of some fuels result in the production of undesirable constituents, such as NOx (nitrogen oxide (NO) and nitrogen dioxide (NO2)), in exhaust emissions.
One approach that has been used to reduce NOx emissions is to reduce the equivalence ratio (equivalence ratio is the actual ratio of fuel and air in the fuel-air mixture to the stoichiometric ratio of the fuel-air mixture) of the fuel-air mixture directed to the combustor. A fuel-air mixture with an equivalence ratio less than 1 is called a lean fuel-air mixture (lean fuel). While lean fuel reduces NOx emissions, government regulations that restrict the amount of NOx and other undesirable gas turbine emissions continue to tighten. It is known that catalytic combustion can provide further reduction in gas turbine engine NOx emissions. In catalytic combustion, a portion of the fuel-air mixture may be combusted in a catalyst system included in the fuel injector. While fuel injectors utilizing lean catalyst systems may reduce NOx emissions, these systems may require a relatively uniform fuel-air mixture (having variation in fuel to air ratio of less than about±3% in some cases) to be directed to the catalyst system. Providing a uniform fuel-air mixture may require additional components that may increase the cost of the gas turbine engine.
An alternate method of reducing NOx emissions, while alleviating some of the deficiencies of a lean catalyst, is to direct a rich fuel (fuel-air mixture with an equivalence ratio greater than 1) through a rich fuel catalyst system (rich catalyst). The construction and composition (such as the catalyst used, etc.) of the rich catalyst may be same as a lean catalyst, but may be optimized to catalyze rich fuel. U.S. Pat. No. 6,358,040 ('040 patent) issued to Pfefferle et al. discloses a gas turbine fuel injector incorporating a rich catalyst. In the system of the '040 patent, a rich fuel is directed to the combustor through a rich catalyst to oxidize a portion of the fuel in the mixture flowing therethrough. In the system of the '040 patent, only a relatively small portion of the fuel in the rich fuel is oxidized in the catalyst. The partially combusted mixture exiting the rich catalyst is mixed with additional compressed air to create a mixture lean in fuel. This lean mixture is then directed to the combustor for combustion through a path which is devoid of structures/devices that may cause flame holding and premature inflammation of the mixture.
While the fuel injector of the '040 patent may decrease the NOx produced by the turbine engine, it may have some drawbacks. For instance, the necessity of avoiding flame holding sites in the fluid flow path from the catalyst to the combustor may prevent the use of conventional flame stabilization mechanisms such as air swirlers and bluff body stabilizers. Elimination of these flame stabilization mechanisms may, in some cases, necessitate a longer combustor for complete combustion of the fuel. In addition to increasing the cost of the combustor, a larger combustor may present difficulties in applications where space is at a premium. The present disclosure is directed at overcoming these or other shortcomings of existing technology.